Oxygen bleaching

ABSTRACT

An apparatus for treating lignocellulosic material such as wood pulp with oxygen under pressure as a step in an oxygen bleaching process. The apparatus introduces the lignocellulosic material, oxygen and other reagents into the pressurized vessel continuously, and continuously withdraws the treated product and spent reagents. The apparatus permits the carrying out of a continuous oxygen bleaching process.

United States Patent Jamieson 1 Aug. 28, 1973 OXYGEN BLEACHING 3,587,257 6/1971 Hurter 68/181 R 2,121,324 6/1938 Manley 68/207 X [751 Gem"? 1mm, Hum, 2,730,888 1/1956 Seely et a1. 68/181 R Swede" 3,134,384 5/1964 Rich 68/181 R x Assignee: Canadian 3,575,795 4/1971 Brinkley Ct 81. 68,207 X Montreal, Quebec, Canada FOREIGN PATENTS OR APPLICATIONS [22] Filed: Dec. 10, 1970 166,221 2/1959 Sweden 68/181 R 21 1. N 949 1 App 0 Primary Examiner-John Petrakes Assistant Examiner-Philip R. Coe [30] Foreign Application Priority Data Attorney-Alexander O. McIntosh Jan. 8, 1970 Canada 071726 [57] ABSTRACT [52] US. Cl 68/15, 68/181 R, 68/183,

68/207, 259/4 259/6 An apparatus for treatmg llgnocelluloslc material such [51] Int. Cl. D211: 9/10 as wood pulp with oxygen under Pressure as a step in [58] Field of Search 68/15, 5 c, 181 R, bleaching P The apparatus immdms 68/183 184, 207 the lignocellulosic material, oxygen and other reagents into the pressurized vessel continuously, and continu- 56] References Cited ously withdraws the treated product and spent re- UNITED STATES PATENTS agents. The apparatus permits the carrying out of a continuous oxygen bleaching process. 3,130,076 4/1964 Gruber et al. 68/183 X 1,044,185 11/1912 Jefferson 68/184 13 Claims, 9 Drawing Figures Patented Aug. 28, 1973 I 3,754,417

4 Sheets-Sheet 1 WATE'R INVENTOR Allan Geoffrey JAMIESON Q 4. MW

PATENT AGENT Patented Aug. 28, 1973 4 Sheets-Sheet 2 INVENTOR Allan Geoffrey JAMIESON PATENT AGENT Patented Aug. 28, 1973 4 Sheets-Sheet 3 KESQ mi INVENTOR Allan Geoffrey JAMIESON PATENT AGENT Patnted Aug. 28, 1973 I 3,754,417

I Shoots-Shoot I PUZP F/GI6- Allan Geoffrey JAMIESON Malnim PATENT AGENT OXYGEN BLEACHING This invention relates to a process for bleaching lignocellulosic material with oxygen and to an apparatus for carrying out the process.

Lignocellulosic material is used widely for making paper and related products. The raw material usually is wood and must be treated to remove the lignin that bonds the cellulose fibres together. Normally the initial part of this treatment takes place in a pulp digester. In some cases the pulp from the digester is suitable for paper making without further purification. For many applications, however, the pulp is too dark in colour and must be bleached. Various bleaching procedures are known, the most common employing chlorine or chlorine containing compounds. Recently oxygen under pressure in alkaline medium has come into use as a means for solubilizing the residual lignin of lignocellulosic material. The efficient commercial utilization of an oxygen bleaching process requires means for contacting large amounts of pulp in alkaline medium with oxygen under pressure. Although batch processes are possible there is advantage in employing a continuous process. A continuous process, however, involves the provision of means for the continuous introduction of pulp and reagents into a pressurized reactor and means for the continuous removal of the treated pulp and spent reagents from the pressurized reactor. The apparatus of the present invention provides means for carrying out such a continuous process.

It is thus a primary object of the invention to provide means for the continuous oxygen treatment of lignocellulosic material. Additional objects will appear hereinafter.

The apparatus of this invention comprises a reaction vessel adapted to operate at supra atmospheric pressure, means for supplying oxygen or an oxygencontaining gas to the vessel under supra atmospheric pressure, means for introducing continuously lignocellulosic pulp in aqueous suspension into the pressurized reaction vessel, means for supplying alkaline reagent to the reaction vessel, means for heating the contents of the reaction vessel, means for renewing continuously the interface between the lignocellulosic pulp and the gas, means for controlling the residence time of the lignocellulosic pulp in the reaction vessel and means for withdrawing continuously oxygen treated lignocellulosic pulpfrom the pressurized reaction vessel.

Optionally means may be provided for controlling the partial pressure of the oxygen in the vessel and for recirculating the gas and/or the aqueous solution through the reaction vessel.

The process of treating. lignocellulosic material employing the apparatus of this invention comprises the steps of l. introducing an aqueous lignocellulosic pulp of 3 percent to 30 percent consistency into a reaction vessel in a continuous stream,

2. admixing with the pulp 1 to percent by weight of alkaline material,

3. contacting the alkaline pulp in the reaction vessel with oxygen or an oxygen-containing gas at a pressure in the range 40-250 p.s.i.g., and 100C. to [50C. for a period of 10 to 30 minutes, and

4. withdrawing the oxygen treated pulp continuously from the pressurized reaction vessel, the introduction and withdrawal of the pulp from the reaction vessel taking place continuously while the vessel is maintained under supra atmospheric pressure.

The oxygen-containing gas employed in the process commonly will be air. The use of air requires higher pressures to be used than with oxygen. For example air will be employed at ISO to 250 p.s.i.g. whereas oxygen is effective at pressures of 40 to I50 p.s.i.g. Normally the oxygen consumed will be equivalent to about I percent by weight of the dry pulp, but may be greater.

The lignocellulosic material may be unbleached or partially bleached material. For example, it may have undergone a prior treatment with chlorine.

The alkali reagent commonly employed is sodium hydroxide. Sodium carbonate and calcium hydroxide are alternative alkali reagents.

Apparatus suitable for carrying out the invention are illustrated in the accompanying drawings wherein FIG. 1 is a diagrammatic sectional view of a reactor and associated equipment;

FIG. 2 is a diagrammatic sectional view of an air mixed reaction vessel and associated equipment;

FIG. 3 is a diagrammatic sectional view of a reactor employing perforated trays as a mixing device;

FIG. 4 is a diagrammatic sectional view of a reactor employing a draft tube air lift for mixing the reactants;

FIG. 5 is a diagrammatic plan view of a toroidal form of reaction vessel;

FIG. 6 is a sectional diagrammatic view along line A-A of FIG. 5;

FIG. 7 is a sectional diagrammatic view of an agitator suitable for use in the reaction vessel of FIG. 5;

FIG. 8 is a diagrammatic plan view of another toroidal form of reaction vessel, and

FIG. 9 is a sectional diagrammatic view of another type of reaction vessel.

Referring to FIG. 1 a pulp washer is shown at 1. Pulp from the washer passes by line 2 to high density pump 3 which pumps it to hopper 4. Hopper 4 feeds the pulp to rotary pocket valve 5. Pocket valve 5 introduces the pulp into pressurized reaction vessel 6. Reaction vessel 6 has three sections. In upper section 7 the pulp is mixed with water introduced from line 8, sodium hydroxide solution from line 9 and steam from line 10. The steam serves to maintain the pulp at the desired temperature. Compressed air enters the upper section through line 11. The alkaline pulp is agitated continuously by propeller mixer 12. From the upper section of the reactor the pulp passes by downcorner 13 to the central section 14 of the reaction vessel. Here the pulp is agitated continuously by propeller mixer 15. From the centralsection the pulp passes by downcorner l6'to lower section 17. Here the pulp is agitated continuously by propeller mixer 18. The compressed air employed in the pulp treatment is exhausted from the reaction vessel through valve 19.

The treated pulp is withdrawn from the pressurized reaction vessel through rotary pocket valve 20 and passes by line 21 through heat exchanger 22 to washer 24. Heat exchanger 22 serves to preheat water fed to the reaction vessel by pump 23 through line 8. Wash water from washer 24 passes partly to waste through line 26 and partly by line 27 to dilute incoming pulp in line 21.

Typical dimensions andconditions for operation of the reactor of FIG. 1 in a 500 ton per day bleach plant when following a first stage chlorination of the pulp are Total height of vessel 37 feet Total diameter of vessel l 1 feet Total mean retention of pulp l minutes Air flow I630 standard cu. ft./min.

Sodium hydroxide 3% based on oven dry pulp Pulp input consistency 12% Consistency in vessel 4% Temperature 130C. Pressure 200 p.s.i.g.

If higher consistency pulp is employed, e.g. 7 percent consistency, the dimensions of the reaction vessel can be reduced while still retaining the pulp for an average of 10 minutes. It is possible to use alternative means for introducing the pulp into the pressurized reaction vessel. Instead of the rotary pocket valve a high density pump can be used. Also the rotary pocket valve at the outlet of the vessel can be replaced by other types of valve such as butterfly or gate.

In FIG. 2 the use of an air mixed reaction vessel is illustrated. Pulp from washer 28 passes by line 29 through high density pump 30 to screw press 31. In screw press 31 the consistency of the pulp is increased, the excess water passing by line 32 to the effluent from washer 28. The high consistency pulp next passes by line 33 to rotary pocket valve 34 by which it is introduced into pressurized reaction vessel 35. The contents of reaction vessel 35 are mixed continuously by compressed air circulated by pump 36. The air used for mixing can of course be replaced by another oxygencontaining gas or by oxygen alone. This air passes by diffusing grid 37 in the bottom of the vessel through the vessel contents and returns to pump 36 by line 38. In line 38 is fitted oxygen analyzer 39 which monitors the oxygen content of the circulating air and when this reaches a predetermined minimum level operates automatic valve 40 to exhaust air from the system. Fitted to the reaction vessel is pressure gauge 41 which monitors the pressure in the vessel and controls valve 42 from air compressor 43 to replenish air exhausted from the vessel. The contents of the vessel are heated by steam introduced from line 44 through valve 45. Valve 45 is controlled by temperature controller 46 which monitors the temperature of the contents of the vessel.

The oxygen treated pulp is withdrawn from the reaction vessel through line 47 by which it passes to screw press 48. Excess liquor is removed from the pulp in the screw press and passes mixed with air through line 49 to separator tank 50. Here the air is separated and returned to the reaction vessel through line 51. Part of the liquor in tank 50 passes to effluent by line 52 carrying the liquor through heat exchanger 53 and exhaust valve 54. Fresh feed water from pump 55 passes through control valve 56, through heat exchanger 53 and by line 57 joins liquor being returned to the reaction vessel. Valve 56 is controlled by consistency meter,

56A. Valve 54 controlled by flow controller 58, governs-the loss of liquor to efiluent. Alternatively a pH controller could be employed to govern the effluent flow. Pump 59 returns liquor from tank 50 to the reaction vessel through valve 60 and line 61.

Valve 60 is controlled by controller 62 which operates the valve to maintain a predetermined level of liquid in tank 50. The alkalinity of the recirculated liquor is maintained at the desired level by caustic solution pumped in by pump 63 through valve 64 and line 65 to line 61. Valve 64 is controlled by pH controller 66 which monitors the pH of the recirculated liquor passing through line 61. The high consistency pulp issuing from screw press 48 passes to rotary pocket valve 67 where it leaves the high pressure section of the system. Vlave 67 is controlled by level controller 68 which acts to maintain the level of the contents of the reaction vessel at a predetermined level. The pulp next passes by line 69 to washer 70. The washings from this washer pass by line 71 partly to effluent and partly are recycled by pump 72 and valve 73 to the washer feed. Valve 73 is controlled by controller 74 which acts to maintain the consistency of the washer feed at the desired level.

In a process utilizing the apparatus of FIG. 2 to bleach 500 tons of pulp per day reaction vessel 35 can have dimensions of 13 feet diameter by 40 feet high permitting a 10 minute retention time. The pulp delivered by washer 28 will have a consistency of 12 percent which will be increased by screw press 31 to 25 percent consistency. The consistency of the pulp in the reaction vessel will be 2 to 6 percent. The caustic concentration will be about 3 percent and the air pressure 200 p.s.i.g. Air recirculation through line 38 will be at 2000 cu.ft./- min. Liquor circulation through line 61 will be at 1700 U.S. gals/min. if consistency of the pulp in the reactor is 4 percent. Steam for heating will be at 200 psi. at the rate of 1100 lbs./hr.

In FIG. 3 is illustrated an alternative form of reaction vessel. This is fitted with a series of perforated trays 75 over which the alkaline pulp suspension flows and through which air is forced. The pulp enters the reaction vessel through line 76 and mixes over the first tray with water, sodium hydroxide and steam entering from line 77. The alkaline pulp is contacted intimately with air rising through perforated tray 75 and then passes by downcomer 78 to the tray below. In this manner the pulp passes to the bottom of the reaction vessel and is withdrawn through line 79. Air enters the reaction vessel through line 80 and after passing through the series of perforated trays leaves through line 81.

In FIG. 4 is illustrated another form of reaction vessel in which a draft tube air lift mechanism is employed to mix the air with the pulp. The pulp enters the reaction vessel through line 82 and flows into upper compartment 83. Here it is mixed with water, steam and sodium hydroxide introduced through line 84. Centrally located in compartment 83 is draft tube 85. Air is introduced into the bottom of tube 85 through air line 86 displacing the pulp upwards in the tube. From compartment 83 the pulp flows by downcomer 87 to central compartment 88 fitted with air lift draft tube 89. Air is circulated from the upper compartment by line 90 to the-bottom of draft tube 89. The pulp from compartment 88 flows by downcomer 91 to lower compartment 92 fitted with air lift draft tube 93. Air to operate air lift draft tube 93 is provided by line 94 which circulates air from the central compartment. Air is exhausted from the lower compartment through line 95. The treated pulp passes from the reaction vessel by line 96.

In'FIG. 5 is shown a toroidal shaped reaction vessel. This vessel is installed horizontally so that the pulp with a pulp-gas interface can travel in a circular path. The pulp enters the reaction vessel by line 97. Water, steam and sodium hydroxide enter by line 98 and air by line 99. The vessel is provided with four propeller agitators 100 positioned so as to pump the pulp around the vessel. A segmented baffle 10] is positioned so as to prevent air from flowing freely around the vessel but per mitting pulp to pass underneath without restriction. The pulp is withdrawn from the vessel through line 102,

the vessel of FIG. 5. This is a finger type agitator having fingers 105 mounted on rotating shaft 106.

bleaching lignocellulosic material with oxygen in a continuous process employing an aqueous pulp suspension which being fluid is readily transferred through the apparatus piping.

and air through line 103. The pumping action of the agitators is such that a pulp fibre will on average make more than one rotation of the vessel before passing through outlet 102. The volume of the reaction vessel is designed to provide a mean residence of minutes 5 for the pulp phase while giving a gas phase volume sufficient to permit easy transfer of air around the vessel. A typical dimension for 500 ton per day using 4% consistency pulp is a toroidal tube of 8 foot internal diameter and a centre locus of 22 feet diameter.

10 In FIG. 6 is shown a cross section of the tube of FIG.

FIG. 7 illustrates an alternative form of agitator for FIG. 8 illustrates an alternative form of toroidal reac- In FIG. 9 is illustrated another alternative design of reaction vessel. The vessel is cylindrical and the direction of flow of pulp is from left to right. Pulp enters through line 114, water, steam and sodium hydroxide through line 115 and air through line 116. The vessel contents are agitated by finger type agitator 117. Pulp is withdrawn from the vessel through line 118; air is exhausted through line 119.

The apparatus of this invention provides a means of What we claim is: 1. An apparatus for the continuous treatment of lignocellulosic material with oxygen or an oxygencontaining gas which comprises 6. means for mixing the lignocellulosic pulp with the oxygen or oxygen-containing gas so as to renew continuously the interface between the lignocellulosic material of the pulp and the gas,

7. means for controlling the level of the aqueous lignocellulosic pulp in the reaction vessel thus controlling the residence time of the lignocellulosic pulp in the vessel, and

8. means for withdrawing continuously oxygen treated aqueous alkaline lignocellulosic pulp from the pressurized reaction vessel.

2. An apparatus as claimed in claim 1 wherein the reaction vessel is a multi-compartment vessel provided with mechanical agitators in each compartment.

3. An apparatus as claimed in claim 1 wherein the reaction vessel is a single compartment vessel provided with a gas dispersing grid located at the bottom of the vessel, said grid being supplied by gas from the means for supplying oxygen or oxygen-containing gas, said grid serving to mix the contents of the vessel. I

4. An apparatus as claimed in claim 3 wherein there is provided means for controlling the partial pressure of the oxygen in the reaction vessel.

5. An apparatus as claimed in claim 3 wherein the means for introducing pulp into the reaction vessel is a rotary pocket valve.

6. An apparatus as claimed in claim 3 wherein the means for withdrawing pulp from the reaction vessel is a rotary pocket valve.

7. An apparatus as claimed in claim 3 wherein the means for heating the contents of the reaction vessel is a source of steam feeding into the vessel.

8. An apparatus as claimed in claim 3 wherein there is provided means for recirculating the oxygencontaining gas through the pulp suspension.

9. An apparatus as claimed in claim 3 wherein there are provided means for separating alkaline aqueous liquor from the lignocellulosic pulp withdrawn from the reaction vessel, and means for recirculating this liquor to the reaction vessel.

10. An apparatus as claimed in claim 1 wherein the reaction vessel is a multi-compartment vessel having a plurality of perforated plate air dispersing members.

11. An apparatus as claimed in claim 1 wherein the reaction vessel is a multi-compartment vessel having draft tube air dispersing members.

12. An apparatus as claimed in claim 1 wherein the reaction vessel is toroidal in shape.

13. An apparatus as claimed in claim 1 wherein the reaction vessel is a singlecompartment tubular vessel provided with means for mechanical mixing of the vessel contents. 

1. An apparatus for the continuous treatment of lignocellulosic material with oxygen or an oxygen-containing gas which comprises
 1. a reaction vessel adapted to operate under supra atmospheric pressure,
 2. means for supplying oxygen or an oxygen-containing gas to the vessel under supra atmospheric pressure,
 3. means for introducing continuously aqueous lignocellulosic pulp into the pressurized reaction vessel,
 4. means for supplying alkaline reagent to the reaction vessel,
 5. means for heating the contents of the reaction vessel,
 6. means for mixing the lignocellulosic pulp with the oxygen or oxygen-containing gas so as to renew continuously the interface between the lignocellulosic material of the pulp and the gas,
 7. means for controlling the level of the aqueous lignocellulosic pulp in the reaction vessel thus controlling the residence time of the lignocellulosic pulp in the vessel, and
 8. means for withdrawing continuously oxygen treated aqueous alkaline lignocellulosic pulp from the pressurized reaction vessel.
 2. means for supplying oxygen or an oxygen-containing gas to the vessel under supra atmospheric pressure,
 2. An apparatus as claimed in claim 1 wherein the reaction vessel is a multi-compartment vessel provided with mechanical agitators in each compartment.
 3. An apparatus as claimed in claim 1 wherein the reaction vessel is a single compartment vessel provided with a gas dispersing grid located at the bottom of the vessel, said grid being supplied by gas from the means for supplying oxygen or oxygen-containing gas, said grid serving to mix the contents of the vessel.
 3. means for introducing continuously aqueous lignocellulosic pulp into the pressurized reaction vessel,
 4. means for supplying alkaline reagent to the reaction vessel,
 4. An apparatus as claimed in claim 3 wherein there is provided means for controlling the partial pressure of the oxygen in the reaction vessel.
 5. An apparatus as claimed in claim 3 wherein the means for introducing pulp into the reaction vessel is a rotary pocket valve.
 5. means for heating the contents of the reaction vessel,
 6. means for mixing the lignocellulosic pulp with the oxygen or oxygen-containing gas so as to renew continuously the interface between the lignocellulosic material of the pulp and the gas,
 6. An apparatus as claimed in claim 3 wherein the means for withdrawing pulp from the reaction vessel is a rotary pocket valve.
 7. An apparatus as claimed in claim 3 wherein the means for heating the contents of the reaction vessel is a source of steam feeding into the vessel.
 7. means for controlling the level of the aqueous lignocellulosic pulp in the reaction vessel thus controlling the residence time of the lignocellulosic pulp in the vessel, and
 8. means for withdrawing continuously oxygen treated aqueous alkaline lignocellulosic pulp from the pressurized reaction vessel.
 8. An apparatus as claimed in claim 3 wherein there is provided means for recirculating the oxygen-containing gas through the pulp suspension.
 9. An apparatus as claimed in claim 3 wherein there are provided means for separating alkaline aqueous liquor from the lignocellulosic pulp withdrawn from the reaction vessel, and means for recirculating this liquor to the reaction vessel.
 10. An apparatus as claimed in claim 1 wherein the reaction vessel is a multi-compartment vessel having a plurality of perforated plate air dispersing members.
 11. An apparatus as claimed in claim 1 Wherein the reaction vessel is a multi-compartment vessel having draft tube air dispersing members.
 12. An apparatus as claimed in claim 1 wherein the reaction vessel is toroidal in shape.
 13. An apparatus as claimed in claim 1 wherein the reaction vessel is a single compartment tubular vessel provided with means for mechanical mixing of the vessel contents. 